Question

Why does HF have a lower boiling point than water even though F is more electronegative than O?

Answer 1

Both HF and water comprises highly electronegative atoms F and O which are capable of forming hydrogen bonds. We can answer this question with the strength of the hydrogen bond forms. Apart from these, any other factors of influence are also possible, we’ll discuss them further in the answer.

Complete answer:
Hydrogen bonding is the strongest intermolecular force in HF and water. The strength depends on the interaction distance, interaction angle and extensivity of the network. We can consider a few factors:
- Number of hydrogen bonding donors and acceptors: Water molecules can accept two hydrogen bonding interactions and donate two hydrogen bonding interactions. Whereas the HF molecule accepts three electron pairs and donates one pair of electrons. Water has a more balanced hydrogen bonding network, which makes the system stable and doesn’t allow it to vaporize. This can be a reason for higher boiling point of water.
- Electronegativity of F and O: F is more electronegative and holds higher electron density. Hence it would not donate hydrogen bond interactions than the lone pairs on Oxygen. From which we can predict that the hydrogen bonding interactions are weaker, hence less boiling point.
- The geometry of the HF molecule is linear and has a zig zag hydrogen bonding network. It forms hydrogen bonds in two dimensions with bond angles of ${116^ \circ }$ . This limits the hydrogen bonding strength. Water has tetrahedral geometry and hence can form 3-dimensional hydrogen bonds.
The above-mentioned reasons can be responsible for higher BP of HF.

Note:
Water has tetrahedral geometry and a 3-dimensional hydrogen bonding structure, then a zig-zag hydrogen bonding structure of HF, rendering the system to vaporize easily. The hydrogen interaction needs to be stable, hence it’ll require a higher temperature to vaporize.